This invention relates to exhaust gas converters of the type used for cleaning the exhaust gases from motor vehicle internal combustion engines.
In order to take into account the fact that, during a cold start, the catalytically-active material of a catalytic converter is below its start-up temperature and must be brought to its operating temperature by the exhaust gases of the engine, the active component of conventional exhaust gas converters may be divided into a comparatively small precatalyst located on the exhaust gas inlet side of the converter and a larger main catalyst located downstream in the direction of exhaust flow. The smaller the mass of catalyst to be heated, the quicker it will reach its start-up temperature. Consequently, the exhaust gases are first allowed to enter a comparatively small precatalyst that will be heated to its start-up temperature comparatively quickly and will thereafter give off heat to the exhaust gases by its exothermic catalytic action. In this way, not only will exhaust cleaning be at least partly effected very shortly after the cold start, but also the main catalyst will be heated comparatively quickly.
A disadvantage of this exhaust gas converter arrangement is the need to provide two catalyst supports having different sizes. For example, when the catalyst supports are made by winding layers of sheet metal, they must be wound to two different sizes. Moreover, the two different supports must be mounted in the housing for the exhaust gas converter in such a way that, first, they can withstand the dynamic stresses imposed, for example, during operation of a vehicle equipped with the converter and, second, appropriate allowances are made for thermal changes of dimensions.
U.S. Pat. No. 5,041,270 discloses an exhaust gas converter arrangement in which an oxidizing catalyst and a reducing catalyst are provided in series on a common porous, monolithic support. This converter arrangement, however, does not include a precatalyst and a main catalyst having the same catalytic reaction, nor does it include a catalytic converter support made by winding layers of sheet metal. Also, this conventional monolithic design principle cannot simply be applied to exhaust gas converters consisting of a precatalyst and a main catalyst on a wound sheet metal support because, at least during the cold-start phase of the engine, the direct conduction of heat from the precatalyst to the main catalyst must be kept as small as possible. Otherwise, the heating time required for start-up of the precatalyst, and hence the time before it can begin to supply heat to the main catalyst, would be prolonged.